It should be fairly easy to assess their condition, take the coolant cap off so there is no pressure in the system and give them a squeeze, if you see cracks then maybe change them?

I would think they should be good for more than 5 years unless they were from a bad batch.

I'm guessing that unlike myself you have never spent a happy few hours of your life by the side of the cold A12 (with thundering traffic whizzing past) waiting for the AA to come and replace a hose and refill your Rad.

You guess right.

So the technical wisdom is ..:

Yes- replace ....to

No - Rubber hoses under high pressure and temperature last for ever and are stronger than steel.

I guess you were indeed unlucky, or had inferior quality hoses. I have just inspected my TR7's excellent original equipment hoses - all supple and no sign of cracking. The temperature can never get really high and the pressure is only about 15 psi - and it has never boiled.